1. Introduction
Thermoelectric power plants that burn the coal to generate electricity produce solid by-products including boiler slag, bottom ash, flue gas desulphurization material and coal fly ash (CFA) [
1]. Amongst them, CFA is the one generated in large quantities and is disposed of in ash dams constructed in the vicinity of the power stations. The dry and wet methods are the most well-known disposal techniques of CFA, which are not effective and highly expensive to maintain. Due to much reliance in coal for power generation, the production of CFA is anticipated to reach more than 1 billion tons by 2030, as its utilization rate does not counterbalance its production rate in some countries [
2]. The utilization rate differs from country to country, with highest reported in Denmark (100%), Italy (100%), the Netherlands (100%), Japan (96.4%), France, Australia and Germany (85%), Canada (75%), and the USA (65%) [
3,
4,
5]. The annual production of CFA in South Africa is around 26 million tons, of which less than 10% is recycled, and the remaining is stored in disposal sites, converting large pieces of land to unusable sites [
6]. The disposal of CFA is an increasing economic and environmental challenge, and therefore, it is imperative that other sustainable methods of managing CFA can be developed [
7].
CFA is largely characterized by metal oxides including SiO
2, Al
2O
3, CaO, Fe
2O
3, MgO, Na
2O and K
2O, and these greatly influence its pH. The pH values of CFA can be categorized into three ranges: slightly alkaline (pH 6.5–7.5), moderately alkaline (pH 7.5–8.5) and highly alkaline (pH > 8) [
8,
9]. The high concentration of soluble salts and the pH values are unfavorable conditions for plant growth, and as such, CFA disposal sites are barren lands with no vegetation. Although some nutrients are available in CFA, it lacks phosphorus and nitrogen, which are highly needed for plant growth. Furthermore, the toxicity of CFA has led to a reduced number of the microorganisms required to fix nitrogen and improve the fertility of the substrate [
10,
11]. Many reports have highlighted the presence of organic and inorganic pollutants in CFA. Trace metals, as well as persistent organic pollutants such as polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs), have been reported in CFA, indicating its contribution to environmental pollution [
12].
The small particles of CFA are easily dispersed into the environment during windy seasons and as such can travel far distances, resulting in air pollution. Toxic elements can leach into groundwaters and pollute the water, resulting in soil, surface and groundwater pollution [
13]. There are some conventional remediation methods to clean up contaminated sites, specifically land contaminated with trace metals. In spite of being efficient, these methods are time consuming, environmentally devastating and expensive [
14]. Phytoremediation techniques are cost-effective and have long-term applicability and aesthetic advantages, hence they have been approved as suitable methods of cleaning up polluted areas and reducing pollutants in the soil and other contaminated sites [
15,
16]. Referring to Zgorelec et al. [
17], phytoremediation techniques do not upset or damage the surrounding environment, nor do they require any additional energy input. They can provide long-term plant cover, which may reduce surface runoff and erosion, as well as minimize the availability and mobility of trace metals in the environment, therefore preventing their transfer to subsequent links of the food web and the migration of toxic ions into groundwaters [
10,
17,
18].
Phytostabilization and phytoextraction are the most used techniques in phytoremediation, and the difference in their mechanisms is that in a phytostabilization method the plant is able to immobilize the pollutants in the root system, while in a phytoextraction process pollutants are translocated into the shoots of the plant [
17,
19,
20,
21].
Zea mays L. was confirmed as a good phytostabilizer for soils contaminated with Zn and Cd [
22]. Plants with a rapid growth rate, high biomass and high tolerance to toxic metals are ideal candidates for phytoextraction, as they can transfer metals to the harvestable plant parts [
23,
24].
The phytoremediation potential of plants species can be evaluated by determining their translocation factor (TF) and bioconcentration factor (BCF) [
25]. The BCF is an index of the capacity of the plant to accumulate metals into its roots with respect to its concentration in environmental matrix, as shown in Equation (1) (
Section 2.6) [
11,
14,
19,
26]. A plant with a BCF value greater than 1 is considered as a good candidate for phytostabilization [
26,
27]. The TF index indicates the ability of the plant to translocate metals from the roots to its aerial parts, as shown in Equation (2) (
Section 2.6), and values above 1 indicate potential for phytoextraction [
10,
14,
16,
28]. The practical application of the phytoremediation process considers the use of plant species that do not require high maintenance and can grow and survive in harsh conditions for longer periods. Despite the potential of the
Helichrysum genus for phytoremediation processes, only a few plants from this family have received attention [
16,
29,
30].
Species including Helichrysum italicum, Helichrysum candolleanum, Helichrysum decumbens and Helichrysum splendidum (Thunb.) Less. have demonstrated potential for phytoremediation of contaminated soils, but no work has been reported for restoration of CFA deposits using H. splendidum (Thunb.) Less. H. splendidum (Thunb.) Less. is a perennial plant that can grow effortlessly in adverse conditions such as hot weather conditions, wetlands and in soils polluted with toxic metals and therefore is an ideal candidate to remediate South African sites polluted with CFA. The aim of this paper is to clarify the potential of phytoremediation using H. splenididum (Thunb.) Less.
2. Methodology
2.1. Sampling Site Description and Physicochemical Parameters of CFA and Soil
A CFA sample was collected from the Eskom, Hendrina power station located in Mpumalanga Province (26°2′ S 29°36′ E). Soil samples were collected some distance away from the ash dams around the power station and were used as controls. Portions of the samples were air-dried, homogenized and stored in glass bottles until further analysis. The pH and electrical conductivity (EC) of the samples were determined by weighing 5 g of CFA or soil, and transferred transferring to 50 mL centrifuge vials and added 10 mL of accurately measured deionized water to each sample. The mixtures were placed on a Labcon platform shaker (Laboratory Marketing Services CC, Maraisburg, South Africa) at 200 rpm for 12 h and then centrifuged for 10 min at 5000 rpm. A pH/conductivity combimeter (Orion Star Series Meter Thermo Fischer Scientific Inc., Beverly, MA, USA) was used to measure the pH and EC of the samples. The Walkley and Black method (chromic acid titration) was used to determine the total organic carbon (TOC) content of the samples, where 0.16 M of a potassium dichromate (K
2Cr
2O
7) standard, 0.50 M ferrous sulphate heptahydrate (FeSO
4.7H
2O) solution and 0.025 M of an ortho-phenanthroline indicator were prepared [
31]. An amount of 0.50 g of the soil or CFA was weighed for each sample and transferred to a 250 mL conical flask. An amount of 10 mL of the K
2Cr
2O
7 was added to each sample, followed by the addition of 20 mL of concentrated sulfuric acid, and the mixture was left to stand for 30 min in the fume hood. Then, 200 mL of deionized water was added to the mixture, followed by 10 drops of ortho-phenanthroline indicator, and the solution was titrated using the FeSO
4.7H
2O solution.
2.2. Pot Trials
Helichrysum splendidum (Thunb.) Less. plants of the same age (twelve weeks), purchased from Random Harvest Nursery (Pretoria, South Africa), were used for the pot trials. The experiment was conducted in an open space under natural conditions during the period from March to June 2023. Plants were transplanted into 4-L plastic pots, with a 20 cm bottom diameter, and were filled to a depth 18 cm with CFA or soil. The experiment included 15 CFA-grown plants and 3 untreated plants growing in the collected soils were used as controls. The plants were watered thrice a week using tap water and were monitored for 14 weeks. Using a measuring tape, the lengths of the longest stems, from the root–stem junction to the leaf apex, were measured every two to three weeks for a period of fourteen weeks to evaluate plant growth.
2.3. Gas-Exchange Measurements
Gas-exchange measurements were used to evaluate the physiological properties of the plants, including CO2 assimilation or photosynthetic rate (A), transpiration efficiency (E), stomatal conductance to water vapor (Gs) and intercellular CO2 (Ci) on a leaf.
Photosynthetic measurements were taken on fully expanded young trifoliate leaves at the same position from the apex for triplicate plants per treatment (
n = 3) and between 08:00 and 11:00 am at days 25 and 40 after planting using a portable infra-red gas analyzer photosynthesis system (Li-6400XT, Li-COR instrument, Nebraska, Lincoln, OR, USA) as described by Makoi et al. [
32]. Leaves were allowed to acclimate to the light environment in the chamber for 4 to 5 min before each measurement was taken. The instrument was calibrated to the following conditions in the leaf chamber before use: light intensity 1000 μmol photons m
−2 s
−1, reference CO
2 concentration 400 ppm, flow rate 400 μmol s
−1, leaf temperature 25 °C and a relative humidity of 44%.
2.4. Plant Harvest
Following the fourteen-week period during which the pot trials were conducted, each entire plant was uprooted from the pot and thoroughly cleaned up with tap water to eliminate any remaining soil or CFA, then rinsed first with 2% HNO3 and finally with distilled water. Plants were separated into roots and shoots and allowed to air dry. Once dried, the plants were milled using a ball miller (BM500, Anton Paar, Midrand, South Africa) at 15 Hz for 15 min, sieved (<2 mm) to a homogenous sample and stored at room temperature until further use. Soil and CFA samples from the pots, after harvest, were also collected in plastic centrifuge vials and allowed to air dry for further analysis.
2.5. Metal Content Analysis
Acid digestion was performed on control soil and CFA samples (before and after the pot trial experiment), a certified reference material (CRM) and plant parts in order to conduct a full elemental analysis using inductively coupled plasma mass spectrometry (ICP-MS). Method validation was conducted using the CRM (SRM 1944, New York Waterway Sediments, NIST, New York, USA). The shoots of H. splendidum (0.25 g) were accurately weighed and transferred into a conical flask, and 5 mL of 65% HNO3 w/w was measured and added into each sample. The mixtures were placed on a hot plate at 110 °C for 30 min, and the resulting digests were diluted to 25 mL using deionized water. For the soil, CFA and CRM samples, a combination of 65% HNO3 and 32% HCl was used. First, we accurately weighed the samples (0.25 g), then added 20 mL HCl:HNO3 (5:15) and heated for 30 min and then added 2 mL hydrogen peroxide (w/w) and further heated for extra 15–30 min. The resulting digests were also diluted to 25 mL using deionized water. The mixtures were then filtered using micro-filters and stored in centrifuge vials until further analysis. The samples were further diluted to a total volume of 10 mL by combining 100 µL of sample and 9.9 mL of deionized water.
ICP-MS was used for the analysis and determination of metal concentrations in the CFA and control soil before and after the pot trial experiment as well as in the CRM and plant materials. The calibration of the instrument was conducted using standards prepared from a 1000 mg L−1 multi-elemental standard stock solution (Fluka A.G., Buchs, Switzerland). The working standard solutions with concentrations of 200, 400, 600, 800 and 1000 µg L−1 were prepared by dilution of the stock. The instrument’s operating parameters were set as follows: nebulizer flow rate (0.88 L min−1), ICP RF power (1500 W), auxiliary gas flow rate (1.2 L min−1), plasma gas flow (18 L min−1) and sample uptake rate (1.6 mL min−1).
2.6. Phytoextraction and Phytostabilization Potential of the Plant
Two indices were used to assess the phytostabilization and phytoextraction potential of trace metals in the plant parts, Bioconcentration Factor (BCF) and Translocation Factor (TF), using Equations (1) and (2) [
25,
33].
2.7. Statistical Analysis
The data were subjected to analysis of variance (ANOVA) to compare means of the treatments using the STATISTICA program (version 10.1). Where there were differences, the Duncan’s multiple range test was used to separate the means at p ≤ 0.05.
4. Conclusions
This study was able to highlight the potential of H. splendidum for phytoremediation of CFA-polluted sites in South Africa. The plants did not show high BCF and TFs values for all elements, but some of them, such as As, Cu and Zn, were greater than 1 in CFA-grown plants, which can make the plant species a good phytoextractor. This study showed an even distribution of metal contents in the soils and plant tissues, and it also showed low TFs for most elements, which also makes the plant species an attractive potential for the phytostabilization of trace metals. Although the CFA medium had restricted growth of some plants as compared to the control soils, the results showed an interesting removal efficiency ranging between 18.0 to 56.7%, with the highest values reported for Cu and Zn. From the obtained results, it can be concluded that H. splendidum has the potential to be used for plant-based remediation of CFA-polluted sites. The technique can be used to reduce the concentrations of trace metals from the CFA, and the CFA can become less toxic and more suitable for plantation. The observed gas-exchange parameters were very similar in both control soil and in the CFA substrates. Further studies of H. splendidum for different CFA treatments need to be conducted to enhance the plant’s phytoremediation capabilities. In summary, the metal concentrations in the CFA-grown plants were almost comparable with those in the soil-grown control plants, importantly the Cr, Zn and Mn concentrations were greater in the shoots for soil-grown control plants compared to the CFA-grown plants. Mn had a high concentration in the roots in both medium-grown plants. The results showed that H. splendidum was able to extract all metals from the substrates to its tissues, rendering it good a phytostabilizer and phytoextractor candidate for trace metals.